1. Field of the Invention
The present invention relates to a method for manufacturing a thin-film transistor using a crystalline silicon film.
The invention also relates to a process to crystallize or improve the crystallinity of an amorphous silicon film or a crystalline silicon film formed on an insulating substrate such as a glass substrate by laser annealing.
The invention also relates to threshold voltage control on a thin film transistor that is formed by using a crystalline silicon film.
2. Description of the Related Art
In recent years, extensive studies have been made of the techniques of crystallizing or improving the crystallinity of an amorphous silicon film or a crystalline silicon film (a silicon film that is not a single crystal but is polycrystalline, microcrystalline, or in some other form of crystallinity), formed on an insulating substrate such as a glass substrate by laser annealing.
Having a large mobility, a crystalline silicon film formed through laser annealing is widely used in a monolithic liquid crystal electrooptical device etc. in which thin-film transistors (TFTs) for a pixel region (pixel driving) and driver circuits are formed by using the crystalline silicon film on a single glass substrate, for instance.
On the other hand, because it can provide high productivity and hence is superior from the industrial viewpoint, a laser annealing method is used by preference. In the laser annealing method, a high-power pulse laser beam emitted from an excimer laser or the like is processed by an optical system to assume a several centimeter square spot or a linear shape of several millimeters in width and several tens of centimeters in length on an irradiation surface and the irradiation surface is scanned with the laser beam (the laser beam illuminating position is moved relatively to the irradiation surface).
In particular, the use of a linear laser beam is advantageous in obtaining high productivity, because in many cases the entire irradiation surface can be subjected to laser light irradiation by scanning in only one direction that is perpendicular to the beam longitudinal direction in l o contrast to the case of using a spot-like laser beam which requires two dimensional scanning.
A TFT using a crystalline silicon film, if a crystalline silicon film constituting the channel forming region is intrinsic, has a general tendency that the threshold voltage is slightly shifted to the negative side of 0 V and the current-rising voltage is about xe2x88x922 to xe2x88x924 V in the case of an n-channel transistor. As a result, there is a marked tendency that the TFT has a normally-on state (it is in an on state even if the gate voltage is 0 V).
When a TFT having a normally-on state is used as a switching element, for instance, current flows through it even if the gate voltage is 0 V. The gate voltage needs to be always biased to the positive side to render the switch in an off state. Therefore, a circuit using such TFTs has various problems such as high current consumption and the necessity of a circuit for applying a bias voltage.
To solve the above problems, threshold voltage control is conventionally performed in which even in the case of producing an n-channel TFT, a crystalline silicon film constituting a channel forming region is doped with a p-type impurity, for instance, boron, to shift the threshold voltage to the positive side. A resulting TFT has a normally-off state (it is off when the gate voltage is 0 V). However, causing an increase in the number of manufacturing steps, the threshold voltage control is a factor of preventing reduction in manufacturing cost.
An object of the present invention is to shift the threshold voltage of a TFT using a crystalline silicon film to the positive side, thereby causing an n-channel TFT to exhibit a normally-off state.
Another object of the invention is to reduce the S-value and increase the mobility.
To attain the above objects, according to a first aspect of the invention, there is provided a manufacturing method of a semiconductor device comprising a first step of laser-annealing a non-single-crystal silicon film that is formed on a substrate having an insulating surface in a hydrogen-inclusive atmosphere; and a second step of forming an insulating film to become a gate insulating film on the non-single-crystal silicon film, the first and second steps being performed consecutively.
In the above manufacturing method, it is preferred that the non-single-crystal silicon film not be exposed to the air between the first and second steps.
According to a second aspect of the invention, there is provided a manufacturing method of a semiconductor device comprising the steps of preparing a consecutive processing apparatus having a laser irradiation chamber, a substrate transfer chamber, and a processing chamber, the respective chambers being airtight; laser-annealing a non-single-crystal silicon film that is formed on a substrate having an insulating surface in a hydrogen-inclusive atmosphere in the laser processing chamber; transferring the substrate to the processing chamber via the substrate transfer chamber; and forming an insulating film to become a gate insulating film on the non-single-crystal silicon film in the processing chamber.
In the above manufacturing method, it is preferred that the insulating film be a silicon nitride film or a multi-layer film including a silicon nitride film.
It is preferred that the multi-layer film include a silicon oxynitride film formed on the non-single-crystal silicon film and a silicon nitride film formed on the silicon oxynitride film.
It is preferred that the multi-layer film include a silicon oxide film formed on the non-single-crystal silicon film and a silicon nitride film formed on the silicon oxide film.
It is preferred that the multi-layer film include a first silicon nitride film formed by nitrifying a surface of the non-single-crystal silicon film and a second silicon nitride film formed on the first silicon nitride film.
In this specification, the term xe2x80x9cconsecutivexe2x80x9d means that there is no step that changes the composition, film quality, shape, or structure of a non-single-crystal silicon film that has just been subjected to the first step, between the first and second steps.
Therefore, providing, between the first and second steps, a substrate transfer step, an alignment step, a slow cooling step, a step of heating a substrate to a temperature suitable for the second step, or a like step is within the scope of the term xe2x80x9cconsecutivexe2x80x9d as used in this specification.
On the other hand, providing, between the first and second steps, a step of exposing a non-single-crystal silicon film to a particular atmosphere (for instance, an oxidizing atmosphere) that changes its film quality, a heating step (for instance, a heating step intended for hydrogen removal, or a heating step performed in an oxidizing atmosphere or the like) to intentionally change the film quality of a non-single-crystal silicon film, an ion doping step, a film formation step, an etching step, a plasma processing step, a coating application step, or a like step is not included in the scope of the term xe2x80x9cconsecutivexe2x80x9d as used in this specification.
In the invention, in crystallizing or improving the crystallinity of a non-single-crystal silicon film by laser annealing, the non-single crystal silicon film is irradiated with laser light in a hydrogen-inclusive atmosphere.
A TFT formed by using a crystalline silicon film that has been obtained by laser-annealing a non-single-crystal silicon film in a hydrogen-inclusive atmosphere exhibits a positive threshold voltage shift of about 2 to 4 V and a current-rising voltage of about 0 V or larger, in both cases of an n-channel and p-channel TFTs. The reason for these phenomena is unknown.
Therefore, the conventional step of controlling the threshold voltage by boron doping can be eliminated.
There is a tendency that the positive shift of the threshold voltage becomes larger as the hydrogen concentration of an atmosphere in the laser annealing step increases. Therefore, it is possible to control the degree of the threshold voltage shift by the hydrogen concentration of the atmosphere.
In this manner, the threshold voltage control can be performed in a TFT manufacturing process without introducing a new step. Therefore, compared to the conventional threshold voltage control by boron doping, the TFT manufacturing process can be simplified and the manufacturing cost can be reduced.
In addition, even if the threshold voltage is shifted by laser annealing in a hydrogen-inclusive atmosphere, the current-rising performance in the current-voltage characteristic of a TFT, which is represented by the S-value (V/decade), shows almost no change; that is, the shift of the threshold voltage causes almost no increase in S-value (almost no deterioration in rising performance). That is, threshold voltage shift causes almost no deterioration in the current-rising performance at the time of switching of a TFT.
Thus, by performing laser annealing in a hydrogen-inclusive atmosphere, a non-single-crystal silicon film can be crystallized or improved in crystallinity, as well as the threshold voltage of a TFT formed by using a resulting crystalline silicon film can be shifted to the positive side without increasing the S-value. Therefore, a TFT that shows superior current rising performance at the time of switching and hence exhibits a normally-off state can be obtained without increasing the number of manufacturing steps.
Further, the above laser annealing in a hydrogen-inclusive atmosphere is employed as the first step, and a step of forming an insulating film to become a gate insulating film on the non-single crystal silicon film is employed as the second step. By performing the first and second steps consecutively, the insulating film to become the gate insulating film can be formed in a state that very little change has occurred in the non-single-crystal silicon film after the first step.
Therefore, the removal of hydrogen, which was introduced in the non-single-crystal silicon film during the laser annealing in a hydrogen-inclusive atmosphere and neutralizes or compensates for dangling bonds of silicon, can be reduced to a very low level, whereby hydrogen can be confined in the channel forming region of the silicon film more effectively.
As a result, the positive threshold voltage shift is increased. Further, the mobility is increased because the number of dangling bonds in the channel forming region is decreased.
Further, the boundary between the channel forming region and the gate insulating film is given superior characteristics, leading to a reduction in S-value.
In addition, avoiding exposure to the air of the non-single-crystal silicon film after the first step is effective in preventing formation of an oxide film on the non-single-crystal silicon film and adhesion of impurities thereto.
As a result, the degree of formation, at the boundary, of an oxide film and impurities, which are factors of generating traps, can be reduced, whereby a reduction in S-value and an increase in mobility can be obtained, the possibility of occurrence of the threshold voltage instability caused by impurity ions can be reduced.
To perform the first and second steps consecutively, i.e., without exposing a non-single-crystal semiconductor film to the air, it is effective to use a multi-chamber consecutive processing apparatus in which a chamber for performing laser annealing and chambers for forming an insulating film are connected to each other via an airtight substrate transfer chamber.
Further, because of an enhanced hydrogen confining effect, it is effective to form a multi-layer film including a silicon nitride film on the non-single crystal film as the gate insulating film.
It is preferred that the multi-layer film containing a silicon nitride film be formed such that a silicon nitride film is formed on a silicon oxide film, on a silicon oxynitride film, or on a silicon nitride film obtained by nitrifying the surface of the non-single-crystal semiconductor film.
The insulating film may be constituted only of a silicon nitride film. However, in this case, the threshold voltage of a resulting thin-film transistor is less stable with respect to a temperature variation as compared with the case of using the multi-layer film.
To perform laser annealing in a hydrogen-inclusive atmosphere, a laser annealing apparatus is used which performs laser annealing on a non-single-crystal silicon film in a laser irradiation chamber capable of atmosphere control, and which is equipped with a means for supplying at least hydrogen to the laser irradiation chamber.
It is preferred that the hydrogen-inclusive atmosphere is a mixed gas of hydrogen and air or an inert gas such as nitrogen, helium, or argon.
It is preferred that the hydrogen-inclusive atmosphere contain hydrogen at more than 1 ppm at the atmospheric pressure. It is even preferred that the hydrogen content be more than 0.1%. It is best that the hydrogen content be more than 1%.
In particular, it is preferred that the purity of each of hydrogen and an inert gas that constitute the hydrogen-inclusive atmosphere be more than 99.9% (3 Ns) and less than 99.99999% (7 Ns). By using the atmosphere constituted of gases whose purity is in this range, it is possible to produce crystalline silicon films that are stable in film quality and TFT characteristics. If the purity of hydrogen and an inert gas that constitute the atmosphere is lower than 3 Ns, the film quality and the TFT characteristics likely become unstable due to impurities in the atmosphere, such as carbon, water, and hydrogen carbides. On the other hand, even if the purity is higher than 7 Ns, no remarkable improvement is found, though a cost increase is incurred. This is the reason why a purity level higher than 7 Ns is not preferred.
The laser annealing may be performed at the atmospheric pressure. However, it is preferred that the laser annealing be performed at a pressure lower than the atmospheric pressure, particularly at 0.01 to 700 Torr, because the degree of roughness of the top surface of a crystalline silicon film or the entire film due to plural times of irradiation with pulse laser beams can be lowered. That is, with improved resistance to pulse laser beams, a resulting film is low in the degree of roughness. If the laser annealing pressure is higher than 700 Torr, the degree of roughness of a resulting film is practically same as the case of the atmospheric pressure. On the other hand, if the pressure is lower than 0.01 Torr, almost no threshold voltage shift due to the use of the hydrogen-inclusive atmosphere is obtained.
It is preferred that an irradiation surface is scanned with a laser beam whose sectional shape on the irradiation surface is spot-like or linear.
Further, it is preferred that a pulsed laser is used as a laser beam light source.
In another aspect, a manufacturing method of a semiconductor device includes introducing impurities in a semiconductor layer comprising silicon formed over a substrate, and scanning the substrate having the semiconductor layer with a pulsed linear laser beam having an elongated cross section by moving the substrate in a direction perpendicular to an elongation direction of the cross section at a fixed speed at an atmospheric pressure. One point of the substrate is irradiated with 20 to 40 pulses of the laser beam.
In a further aspect, the method includes forming a crystalline semiconductor layer comprising silicon over a substrate, and forming a gate electrode adjacent to the semiconductor layer with a gate insulating film interposed therebetween.
In a still further aspect, the method includes scanning a semiconductor layer comprising silicon over a substrate, with a pulsed linear laser beam having an elongated cross section by moving the substrate in a direction perpendicular to an elongation direction of the cross section at a fixed speed. One point of the substrate is irradiated with 10 to 50 pulses of the laser beam.